The present invention relates to a method of operating a gas-turbine group with cooling by air extracted from the process.
In gas-turbine groups of the more recent generation, the cooling of the units subjected to high thermal loading plays a central role. In principle, the aim is to extract air from the process at a suitable point, to feed this air to the units to be cooled and to then feed it back again at a suitable point into the cycle of the gas-turbine group. In this type of cooling, the effective mass flow of the plant is always diminished, which invariably leads to an efficiency loss of the same. This is in principle connected with the fact that, in these gas-turbine groups, there is as a rule no excess air which could be diverted without loss of output and efficiency. The initial situation with regard to cooling the units subjected to high thermal loading is accentuated if the gas-turbine group is in combination with a steam cycle, that is if it is operated as a combined-cycle plant, since the exhaust gases from the last turbine of the gas-turbine group, which furnish the actual thermal potential for the formation of steam, must have temperatures of over 600.degree. C. in order to obtain as much steam of superior quality as possible. It is obvious that this exhaust-gas temperature induces upstream the temperature of the hot gases from the combustion chambers, whereby the same accordingly have to be designed for intensive cooling. As far as is apparent, compromises invariably have to be made for such cooling, which compromises affect either the cooling capacity or the efficiency of the plant.